Method of distinguishing material characteristics in a well bore for controlling photograph taking in the well bore

ABSTRACT

A combination of magnetics and a special geometric environment is employed to distinguish whether an instrument is positioned within a pipe formed of magnetizable or non-magnetizable material, and is the basis, in part, for a method and an apparatus used in controlling picture taking at the bottom of a well bore. A coil is energized repetitively in a circuit in which the voltage across the coil, or the current that flows through the coil, can be measured. Whether the coil is in a magnetic or non-magnetic environment is determined by the magnitude of the change in coil voltage or current.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of earlier application Ser. No.387,598, filed 06/11/82, now U.S. Pat No. 4,527,122, which applicationwas a division of application Ser. No. 119,171, filed 02/06/80, now U.S.Pat No. 4,365,197, which application was a continuation-in-part ofapplication Ser. No. 892,284, filed 03/31/78, now abandoned.

TECHNICAL FIELD

This invention relates to methods and means for determining whether ametallic enclosure, in particular a drill pipe, is made of magnetic ornon-magnetic material, and it relates to an instrument having thatcapability for controlling illumination of the light source in thephotographing of compasses at the bottom of drilled wells.

BACKGROUND ART

While not limited to that application, the invention is particularlyuseful in connection with the taking of photographic pictures of acompass, or inclinometer, or both, at the bottom of a well bore. Welldrillers can control the direction of deep wells by control of thedrilling tools. But control is accomplished in terms of adjustment tochange from current direction as drilling proceeds. That means that thedriller must know the wells' current direction from time to time.Current direction is determined by lowering a compass inclinometer tothe bottom of the well and then photographing the compass assembly torecord its direction in azimuth and inclination. The task ofphotographing a compass at the bottom of a well is both complicated andexpensive.

The compass needle is acted on by the earth's magnetic field. To permitthat, the compass must be disposed in a non-magnetic section of pipe atthe time that the photograph is taken. Current practice is to include ashort length of non-magnetic pipe at or near the lower end of the drillpipe. That length of pipe is ordinarily made of Monel and it is called a"collar." The compass must be disposed in that section when it isphotographed. Care must be taken to ensure that the compass assembly isnot in motion at the time that the photograph is taken. Current practiceis to include a detector which detects absence of motion and a means forprecluding exposure of the film for some selected time interval measuredfrom the last action which could cause movement of the compass needle.

To prevent premature exposure, it has been necessary to ensure thatinstrument motion does not stop until the instrument has reached theposition at which the photograph is to be taken. To overcome thatrequirement, a conventional clock has been used so that photographtaking occurs at some fixed time after the clock is started at the wellhead. That solution is subject to failure if, for any reason the timerequired to lower the instrument is other than the predicted time.

This invention relates to the problem of ensuring that the compass is inthe non-magnetic collar and to the problem of postponing picture takinguntil the compass needle has "settled down" before the film is exposed.To lower the apparatus into, and to retrieve if from, a deep well isvery costly. It is important to be able to determine reliably and with ahigh degree of certainty whether the compass is, or is not, in thenon-magnetic section of the drill pipe.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide an imporved method forcontrolling photograph taking in wells.

A related object is to ensure that picture taking is accomplished at aselected point in a well, and, in this connection, to provide animproved apparatus for detecting the presence of metal and,particularly, whether that metal is or is not a magnetic section of apipe.

It is an object of the invention to provide an improved method relatedto deep well photography by which to determine whether there is or isnot a magnetizable body in the vicinity of a selected spacial positionand, in particular, to determine whether a given point along a well iswithin a pipe of magnetic or non-magnetic material.

These and other obJects and advantages of the invention are realized, inpart, by the provision of a coil which is energizable to create amagnetic field and of a means for holding the coil within an enclosuresuch that the field of the coil will be substantially confined by theenclosure, in the event that the enclosure is made of non-magnetizableconductive material, and which will induce substantial circulatingcurrents, enough to present a heavy load permitting current flow in thecoil in excess of the current that would flow if the coil was located infree space, in the event that the enclosure is made of magnetizable butelectrically conductive material.

That kind of a coil is used in conjunction with a means for measuringthe voltage across, or the current through, the coil when energized. Thevoltage or current is indicative of whether the coil is disposed in amagnetic or non-magnetic pipe or other enclosure, and it is used todevelop an indicating signal which indicates the nature of theenclosure. In preferred form, the coil is energized through a transistorwhose control electrode is subjected to a periodically varying voltagewave form.

It is a feature to change from analog to digital measurement byswitching if the voltage or current excursion exceeds some value thatindicates that the coil is in one kind of enclosure and not the other.It is another feature to house the coil in a non-magnetic casing inwhich case the instrument can detect when it is in a magneticenvironment and in a non-magnetic environment.

It is a further feature to use the indicating signal to controlinitiation of interrelated timing circuits. cuits.

These and other features and advantages of the invention will becomeclear upon a reading of the specification which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic representation of an instrument in which theinvention is embodied in a section of drill pipe shown in cross-section;

FIG. 2 is a diagram illustrating part of the operation of the inventionwhen detecting pipe of magnetic material;

FIG. 3 is a diagram illustrating part of the operation of the inventionwhen detecting pipe of non-magnetic but metallic material;

FIG. 4 is a circuit diagram of a preferred embodiment of the invention;and

FIGS. 5 and 6 are diagrams of the wave shapes at the terminals of thesensing transistor when the sensing coil is in non-magnetic pipe, in thecase of FIG. 5, and in magnetic pipe in the case of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, the numeral 10 designates a section of drill pipe including afragment of the lower end of a magnetic steel pipe 12 and a fragment ofthe upper end of a pipe 14 of non-magnetic metal. The two areinterconnected at threaded region 16. Disposed within the pipe is aninstrument 18 whose purpose is to photograph the position of a compassassembly on a film.

Sometimes the compass serves as an inclinometer as well as a compass,and in some cases a separate inverted plumbob is added to indicateinclination. Those elements form no part of the invention, nor does thefilm which is placed on one side of the compass and inclinometercombination, or the lamp that is placed on the other side. The inventionis concerned with illumination of the lamp and with illuminating thelamp only when the compass is in the nonmagnetic, metallic portion ofthe pipe, after sufficient time has elapsed to allow the compass needleto become steady.

To perform those several functions, the instrument of the preferredembodiment includes a metal type detector, timers, a power source in theform of batteries, lamp, lamp circuit, a compass and inclinometer, and afilm. In the preferred form, these elements are housed in a non-magneticcase which may be formed of electrically conductive material such, forexample, as Monel metal. The diagram of FIG. 4 includes the batteries,metal type detector, timers, and terminals for connection to the lampcircuit. The metal type detector includes a sensing coil 20 and itsoperation will be explained in connection with FIGS. 2 and 3.

In preferred form, the sensing coil 20 is wound about a straightelongated form, preferably about an elongated ferrite core. It is housedin the instrument case 18 so that the axis of the coil is parallel to,and preferably coincident with, the axis of case 18. The case isarranged so that its axis can be expected to be substantially parallelwith the axis of the drill pipe. The requirement, in the preferredembodiment, is that the axis of the coil be more parallel thantransverse to the axis of the drill pipe. The reason for thatarrangement is explained in connection with FIG. 3.

In FIG. 3, the coil and its core 22 are disposed within a Monel pipe 23.When current flows through the coil, a magnetic field is establishedaround the coil. That field is represented, in accordance withconvention, by closed loops 25, 26, 27 and 28 shown as dashed lineswhich extend through the core, and then as dashed lines which extendthrough the core, and then out at one end and around and back into thecore at the opposite end. No loop crosses any other according toconvention. Since the permeability of Monel is near unity, the fluxlines pass through the pipe walls in a pattern similar to what would beexpected in air. As the flux field builds, the flux lines "cut" the pipewall and that movement of magnetism interacts with the free electrons inthe electrically conductive pipe to cause a motion of those electronsaccording to the right-hand rule. The electrons flow around thecircumference of the pipe so that the pipe acts as a single "shorted"turn secondary winding of large cross-sectional area. The resistance ofthe pipe is low so that it presents a heavy load to the primary winding22 which draws a heavy current. Current flow is represented in FIG. 3 bythe "flow away" and "flow toward" symbols 30 and 32, respectively.

Returning to FIG. 4, the coil 20 is in series with the emitter-collectorcircuit of a transistor 118 and a load resistor 98. The transistor iscut off during part of each cycle so the result is a large currentchange and a large voltage change at the collector of transistor 118when the coil 20 is in Monel pipe.

If the coil 20 was oriented so that its axis was normal to the pipeaxis, the flux direction would be changed by 90 degrees and so would thedirection of the induced current. The current would flow in thelongitudinal direction of the pipe and the pipe would act as a secondarywinding in much lesser degree determined primarily by dissymmetry in themagnetic field.

Sensitivity is also effected by conductivity of the protective case 18.That case must be made of nonmagnetic material so that the magneticfield will not be confined to the case but can reach the pipe.Sensitivity of the instrument is reduced in the sense that a portion ofthe current change in the coil results from current flow in the case.However, it will be seen that that effect does not lessen ability of theinstrument to distinguish steel pipe from a non-magnetic pipe.

Magnetic lines of flux follow the path of least reluctance, and when thecoil 20 is disposed within a pipe 34 of magnetic material, the fluxlines will extend from one end of core 22 through the non-magnetic caseof instrument 18 into and along the magnetic pipe and then back throughthe case of instrument 18 to core 22. There will be less "cutting" ofthe pipe by flux lines. A current will be induced in the pipe wall as itwas in the Monel pipe of FIG. 3, but higher resistance, eddy currentsand other losses and phase stifling effects serve to minimize thatcurrent. Further, the greater magnetic efficiency in the magneticcircuit results in a stronger field, collapse of more flux whentransistor 118 cuts off current, and it creates a greatercounter-electromotive force in coil 20. The result is a lesser currentswing. That is true notwithstanding the circulating current around thecase of instrument 18 and the load it represents.

The end result is that presence of coil 20 within the steel pipe resultsin a magnetic action which produces a counter-electromotive force in thecoil and an opposition to current change which substantially nullifiesthe effect of the instrument case as a shorted turn. There is no suchnullification when the instrument is housed in a non-magnetic pipe socurrent change is large whether it be the result of the instrument caseor the pipe, or both, acting as a shorted turn.

There is another effect to be accounted for. Lenz's Law describes thatcounter-electromotive force has a magnitude that depends upon the rateof change of magnetic flux. At very low frequencies, thecounter-electromotive force opposing flow in coil 20 is very lownotwithstanding that the coil is disposed within the steel portion ofthe pipe. Thus, at very low frequencies, the signal output at thecollector of transistor 118 will be relatively high when the coil ishoused in steel. That signal strength diminishes rapidly when thefrequency is increased. An opposite effect occurs at low frequencieswhen the coil 20 is disposed in the Monel portion of the pipe. At lowfrequencies the flux change is slow and the magnitude of the circulatingcurrent in the "shorted turn" pipe is small. That means that the load issmall and the self-inductance of coil 20 is adequate to limit currentflow. Accordingly, the voltage excursion at the collector of transistor118 is relatively small. However, the signal magnitude increases rapidlywith frequency. In an actual circumstance, the non-magnetic collar hasan inside diameter of about three inches and a wall thickness greaterthan one inch. The instrument case has an outside diameter of about twoinches and a wall thickness on the order of one-quarter inch. The lowerfrequency limit for operation of the instrument will be about 300cycles. At frequencies below that, it becomes impossible to distinguishMonel from steel, or the results are unreliable. As frequency isincreased above 300 cycles per second, the differential in magnitudebetween the signal in steel and the signal in Monel is increased untilsome optimum frequency is reached. For the dimensions given, thatfrequency will fall in the range of 400 to 800 cycles per second. Abovethat, the current variation in coil 20 is diminished when the coil isdisposed in Monel. The effect of self-inductance in the coil overtakesthe effect of the heavy loading so that at about 800 cycles per second,for the values given, the magnitude of the signal is reduced to thepoint where detection becomes difficult and unreliable. Accordingly,there is a frequency window in which the metal type detector must beoperated. That window can be shifted by changing the characteristics ofthe coil 20, but the preferred embodiment employs the component valueslisted below, and a frequency of operation between 400 and 600 cyclesper second. In the particular case shown, the frequency is 500 cyclesper second.

At the upper portion of FIG. 4, line 106 is connected to positiveterminal 104 and power line 108 is connected to ground. The circuitincludes an oscillator 110 whose frequency is relatively low, and itneed comprise no more than a series of three inverters and some feedbackresistors and a capacitor. Its output is connected to the base of NPNtransistor 112. Its collector is connected to the positive line 106. Itsemitter is connected to ground through the series combination of aresistor 114 and an electrolytic capacitor 116. The junction of theresistor and the capacitor is connected to the base of a NPN transistor118 whose collector is connected to the positive line 106 throughresistor 98, mentioned earlier. The emitter of the transistor isconnected through the sensing coil 20 to ground line 108. The output oftransistor 118 is taken from its collector and applied to the base of anNPN transistor 120 whose collector is connected to the positive linethrough a load resistor 122 and to the base of an PNP transistor 124.The emitter of transistor 120 is connected to ground through resistor126, and it is connected to the emitter of a transistor 128 whosecollector is connected to the positive line through load resistor 130.The base of transistor 128 is connected to the junction between tworesistors, 132 and 140, which are connected between the positive lineand the negative line in series, in that order. The emitter oftransistor 124 is connected to the positive line 106 and its collectoris connected through a diode 136 to a line 138. A capacitor 142 and aresistor 144 are connected in parallel between line 138 and the negativeline 108. Another output line 146 is connected from line 138 to the "D"input terminal of a D-type flip-flop 148.

Except for the D-type flip-flop, the circuit thus far described is themetal type detector. Its function is to apply a positive signal to the Dterminal of the flip-flop when coil 20 is located within the Monelsection of the pipe and to apply a negative signal to the D input offlip-flop 148 when the sensing coil 20 is located within a steel portionof the drill pipe. To do that, the output of oscillator 110 is appliedto the base terminal of amplifier 112. Current flowing through thecollector-emitter circuit of the transistor flows through resistor 114to charge capacitor 116. The capacitor discharges through thebase-emitter circuit of the following transistor 118. The result is theapplication to the base of transistor 118 of the voltage that appearsacross capacitor 116. That input voltage causes a corresponding flow ofcurrent in the emitter-collector circuit of transistor 118, except asthat current flow, and counter-electromotive force, develops an opposingvoltage across the sensing coil 20 in the manner previously described.The output signal of that transistor is applied to the base oftransistor 120 whose bias is controlled by current flow throughtransistor 128. The baseof the latter is biased to a fixed value by thevoltage divider formed by the combination of resistors 132 and 140.Current flowing through the transistor 128 flows through resistor 126 inthe emitter circuit of transistor 120. Only when the base voltage of thetransistor exceeds a value determined by the voltage across resistor 126does current flow in transistor 120. The output of that transistor istaken from its collector and applied to the base of switching transistor124 which, when turned on, permits current flow through the diode 136into capacitor 142 so that the output line 138 becomes positive. Thecapacitor is discharged through resistor 144. The effect is that line146 will remain positive while the coil 20 is disposed within the Monelcollar, but the line will return to its negative state when the coil 20is disposed within the steel pipe. In the latter case, the voltage atthe base of transistor 120 is insufficient, in view of the voltageacross resistor 126, to permit current flow through transistor 120 forthe turning on of transistor 124. Accordingly, the charge on capacitor142 will leak off through resistor 144 and line 146 will be returned toa low state and will remain low.

The edge clocked flip-flop 148 initiates a counting and control cyclewhen a high signal is applied to its D input by line 146 from the outputline 138. Clock signals for the digital portion of the circuit aregenerated in an oscillator 150 whose output signals are applied by aline 152 to the clock pins 3 and 13 of JK flip-flops 154 and 156,respectively. Clock signals are also applied to the clock pin CK offlip-flop 148 and to pin 2 of each of three counters, 158, 160 and 162.The counters are cleared and preset when a low signal is applied totheir number 1 pins from the Q-bar output of flip-flop 156. Countingbegins when a high signal is applied to the enable pin, number 9, ofeach of the counters from the Q output of flip-flop 148.

The circuit is arranged so that two counting functions are performed.Counters 158 and 162 are connected together as one counter called theexposure time counter, and counters 158 and 160 are connected togetheras a second counter called the settling time counter. The first of thosecounters is used to control the time during which the camera lightsource, the lamp, is illuminated. The second of the counters introducesa time delay between the time when coil 20 has sensed that it isdisposed within the Monel section of pipe and the time when energizationof the lamp excitation circuit is permitted. Output of the exposure timecounter is taken from pin 15 of counter 162, and the output of thesettling time counter is taken at pin 15 of the counter chip 160. Pin 15of counter chip 162 is connected to the K input of flip-flop 154 and tothe J input of flip-flop 156. The output of counter chip 160 at pin 15is applied to the J input of counter 154.

Turning to the output end of the circuit, the lamp excitation circuit isconnected to terminals X, Y at the right side of FIG. 4. Terminal X isat the collector of a switching transistor 170 whose emitter isconnected to terminal Y through a resistor 172. The base of thattransistor is connected through a resistor 174 to the Q output terminalof flip-flop 154. The transistor 170 is turned on to energize the lampexcitation circuit when the Q output of flip-flop 154 goes high. Q goeshigh at the clock signal if J is high and K is low.

The function of flip-flop 148 is to start the counters. The function offlip-flop 154 is to control turn-on and turn-off of transistor 170, andthe function of flip-flop 156 is to reset the counters after completionof the settling time and the exposure time counts. The exposure countingdoes not begin until the settling time counter has completed its countand has furnished a high signal to the J input of flip-flop 154. Thatmode of operation can be achieved because the counters will not countwhile a low signal is applied to their pin 7. Pin 7 of the exposure timeinterval counter is driven by the Q output of flip-flop 154, and pin 7of the settling time counter is driven by the Q-bar output of flip-flop154.

The digital circuit operates as follows. Oscillator 150 generates clocksignals which are applied to the clock inputs of the several digitaldevices. However, none of them count or change state until a positivesignal appears on line 138 of the metal-type detector circuit and isapplied to the D input pin of flip-flop 148. The flip-flop is thenclocked so that the Q output goes high. That applies an enable signal toeach of the three counter chips. The Q-bar output of flip-flop 148 goeslow, and that signal is applied to the reset pins 4 and 12 of flip-flops154 and 156, respectively. The counter has been cleared previously, sothe count in counter 158 begins at "0." When that counter reachesmaximum count, a high appears at pin 15 and that is applied to the countenable pins 10 of both of the counter chips 160 and 162. Flip-flop 154has been reset. A high appears at the Q-bar terminal, and that isapplied to pin 7 of counter 160 which is thereby enabled. A low isapplied to pin 7 of counter 162 which is connected to the Q output offlip-flop 154. That means that the counter 162 is inhibited. Whencounter 160 reaches maximum count, a high signal appears at its pin 15and that signal is applied to the J input of flip-flop 154. The K inputof that flip-flop, and the J input of flip-flop 156 are low because theyare connected to pin 15 of counter 162 which is inhibited. When counter160 reaches its maximum count, a positive signal is applied to pin J offlip-flop 154 and that flip-flop changes state. A high appears at its Qoutput, thereby turning on the transistor 170 in the lamp excitationcircuit and also enabling the counter 162. When counter 162 reaches fullcount, a positive signal from pin 15 of counter chip 162 is applied tothe K input of flip-flop 154, and the J side of flip-flop 156. Flip-flop154 resets and flipflop 156 sets. Accordingly, the Q output of flip-flop154 goes low to turn off the transistor 170 in the lamp excitationcircuit. The Q-bar output goes high, terminating the signal to counter160. The K input of the flip-flop 156 is at ground potential, so theappearance of a high at the end of the interval count at terminal J offlip-flop 156 changes its state so that a low appears at the Q-baroutput. That low is applied to clear all of the counters.

The circuit of FIG. 4 is arranged so that a photograph will be takenautomatically as soon as the compass photographing instrument ispositioned so that coil 20 is within the Monel collar, and a time haselapsed sufficient to ensure that the compass assembly has stoppedfluctuating.

In a representative circuit, the components may have the followingvalues:

Transistors 112, 120, 118, 170 are type 2N3703;

Transistor 124 is type 2N3703;

D-type flip-flop 148 may be type MC 14013B;

Three counter chips are type MC 14163B;

Flip-flop 154 is type MC 14027B;

Flip-flop 156 is type MC 14584B;

Capacitor 176 has a value of 0.01 mf;

Capacitor 142 has a value of 1 mf;

Capacitor 116 has a value of 2 mf;

Resistors 122, 130 and 144 are 100 K ohms;

Resistors 126 and 134 are 10 K ohms;

Resistor 114 has the value 300 ohms;

Resistor 98 has the value 2700 ohms.

The circuit uses six inverters packaged in chip MC 14069B. Three ofthose are connected in series to form oscillator 110, and the otherthree are connected in series to form oscillator 150. In each case, thefirst inverter in the series is connected in parallel with the seriescombination of a 1 megohm and a 2 megohm resistor. The junction betweenthe two resistors is connected to the junction between the second andthird inverters of the series through a capacitor which, in the case ofoscillator 107, has the value 0.001 mf. The value of the capacitor inoscillator 150 is much larger. Oscillator 150 serves as the clock forthe settling time counter and for the exposure time counter. The lengthof the settling time and exposure will depend upon the characteristicsof the film and of the damping characteristics of the compass. Becauseof that, the value of the clock frequency controlling capacitor 134 isnot specified and the counter-connections (number of counts) are notshown in the diagram. To select them in a given case requires no morethan very ordinary skill. Settling time might be anything up to about1.5 minutes, and the exposure time anything up to forty-five seconds,depending upon which ones are selected of the films and lamps andcompasses that are currently in use.

The coil 20 has an inductance of about 233 microhenries. The corematerial of the coil is magnetic but is non-conductive. The frequency ofoscillator 110 is at or near 500 Hz.

In some applications of the invention, when used as a metal detector, orin conjunction with other instruments, it may be desirable to work withanalog signals, and, for that purpose, the preferred embodiment includesa terminal 202 at the collector of transistor 118. That terminal couldbe positioned at any point where the voltage or current variation incoil 20 can be measured, but the collector position is now preferred.

Experimentation with core materials for the coil has demonstrated thatmagnetic characteristics may vary greatly from sample to sample ofcommercially available ferrite core materials notwithstanding that theyare described by the same or similar specifications. Use of certainpieces of a lot of cores operates to lower the optimum excitationfrequency closer to 300 Hz than to the frequencies and ranges describedabove. Thus, merely by selection of cores it is possible to practice theinvention with optimum results by energizing the core at frequencies inthe range 200 Hz to 800 Hz.

The variation in the magnetic quality of ferrite cores now make itpreferable to employ a core material which can be and is produced withqualties that fall reliably within a narrow range of values. Monel issuch a material. While Monel is a brand name, it is applied to an alloyof nickel, copper and other constituents which are very well known. Inthe oil fields at least, the term "Monel" is used both as a brand nameand as the generic term for alloys of the same general type. All arecalled "non-magnetic" although they exhibit magnetic qualties in somedegree. Substitution of a Monel-type material, a "non-magnetic" butslightly magnetic metal, permits reduction of the core 22 by about halfand lower optimum frequency to the range 275 Hz to 350 Hz. At thepresent time, the preferred embodiment of the invention employs such acore material. The ferrite core is no less useful than previouslydescribed, however, and the preferred excitation frequency remains inthe range 200 to 800 cycles per second.

The preferred mode of detection and the detection circuit remainunchanged. FIG. 5 is a drawing of the voltage wave form at the base,collector and emitter of transistor 118 when the coil is in anon-magnetic metal pipe. FIG. 6 is a drawing of the voltage wave form atthe base, collector and emitter of transistor 118 when the coil 20 is ina magnetic pipe.

In FIG. 5, phase shift resulting from the effect of loss of energy andself induction alters the voltage wave form at time A such that thebase-emitter diode becomes reversed biased. The transistor is cut offand the collector voltage rises to a high value. The coil, which is in anon-magnetic environment, is seen to ring at its natural resonantfrequency to delay the rise in emitter voltage. The ringing decays andemitter voltage rises. At time B, the base-emitter diode bias isreversed to allow conduction. As a consequence, collector voltage falls.

FIG. 6 describes the voltage relationships when the coil 20 is disposedin a magnetic section of pipe. In this case, there is no phase shift atthe cycle beginning sufficient to reverse the bias of the base emitterjunction and the transistor is not switched off. There is no sharp risein collector voltage.

The phase shift in emitter voltage at the beginning of the cycle resultsfrom a combination of energy loss and self induction. Amplitude, andtherefore relative amplitude of base and emitter voltage, is a functionprimarily of loss. Phase shift where there is loss is a function offrequency so the selection of a proper exciting frequency is necessary.

While a preferred embodiment of the invention has been described, otherembodiments of the invention are possible. A list of variations wouldinclude changes in coil configuration, changes in the type ofnon-magnetic material in which the coil was housed, and changes inhousing configuration and, of course, other changes are possible withinthe invention.

We claim:
 1. The method of controlling photograph taking at a selectedposition in a well bore with photographic equipment which comprises thesteps of:(a) lowering the photographic equipment to said positions; (b)initiating a clock timer only when said photographic equipment isdisposed at said position; (c) operating said photographic equipment atthe end of a predetermined interval following initiation of said clocktimer; and (d) comprising the further step of detecting when saidphotographic equipment is disposed at said position by detecting whethermatereial is magnetic or non-magnetic in character at said position. 2.The invention defined in claim 1 in which detection of the character ofthe material surrounding the equipment at said position is accomplishedby observing the effect of said material on a magnetic field emanatingfrom a coil which is supplied with electrical energy having frequencycomponents in the range of 200 to 800 Hz.
 3. The method of controllingoperation of photograph taking in a well bore to ensure that the subjectof the photograph is in a substantially non-magnetic environment at thetime of taking the photograph, which method comprises the steps of:(a)lowering photographic equipment and a photographic subject into the wellwith means for determining whether said photographic subject is in amagnetic or substantially non-magnetic environment; (b) determining withthe aid of said means when said photographic subject is in anon-magnetic environment; (c) operating said photographic equipment tophotograph said subject when the subject is in a non-magneticenvironment; and (d) in which determining whether the environment ismagnetic or non-magnetic is accomplished by observing the effect of saidenvironment on a magnetic field whose intensity is varied such that theresultant variation includes components which vary at a frequency in therange of 200 to 800 Hz.
 4. The invention defined in claim 3 in which thedetermination of whether the environment is magnetic or non-magnetic isaccomplished by observing the effect of said environment on a magneticfield whose intensity is varied such that the resulatnt variationincludes components which vary at a frequency in the range of 300 to 800Hz.